Evaporation is a surface phenomena and refers to the change of phase of a substance from the liquid to the gas phase. Also remember that sunlight is a part of the electromagnetic radiations generated by the Sun and hence is heat energy (radiation is a form of heat transfer and that is why we feel hot when we stand in the sunlight for too long).
When sunlight falls on the pot of water, the water molecules at the surface gain some kinetic energy. Once their kinetic energy is enough to overcome the attractive forces of water molecules, they are able to escape from the water phase and move to the gas phase. If the water is left out for a couple of days, all of it would evaporate and we would be left with the empty pot.
Note that the loss of water through evaporation takes place only when the relative humidity of the air is less than 100%. If the air is 100% saturated with water, no more water vapors can be added to it. At that point, the amount of water lost by evaporation would be equal to water gained.
Hope this helps.
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